1. Field of the Invention
This invention relates to a liquid crystal composition for liquid crystal display. More particularly, it relates to a liquid crystal composition used for a display element employing active matrix mode, and a liquid crystal display element using the same.
2. Description of the Related Art
The liquid crystal display device (LCD) makes possible lower power consumption, smaller type and lighter weight; hence it has been utilized for handy type electronic calculator, digital watch, etc. At present, an active matrix LCD (AM-LCD) such as TFT (thin film transistor), advanced in high precision (high contrast) and colorization, has been expected as a prospective superior product for use in the LCD display method.
As the AM-LCD actuation mode, a nematic (TN) display mode has been employed, wherein the aligning direction of molecules of liquid crystal phase filled between two upper and lower substrates in a cell is twisted by 90.degree.. General conditions sought for the liquid crystal substance used for LCD of TN display mode consist in that the substance exhibits a liquid crystal phase within a temperature range as broad as possible, around room temperature; it is sufficiently stable to environmental factors used; and it is provided with sufficient physical properties for driving the display element. At present, however, a single substance satisfying these conditions has not yet been found; hence several kinds of liquid crystal compounds and if necessary, non-crystalline compounds are blended to prepare a liquid crystal composition having various characteristics, which has been used as a material for LCD.
In the case of AM-LCD, main characteristics sought for the nematic liquid crystal composition used for AM-LCD are as follows:
1) a smaller threshold voltage, PA1 2) a higher specific resistance i.e. voltage retention at service temperature; PA1 3) a suitably larger optical anisotropy (.DELTA.n), etc. PA1 a liquid crystal composition characterized by containing PA1 as a first component, at least one compound expressed by the formula (1): ##STR2## wherein R.sup.1 represents an alkyl group of 1 to 10C; n represents 0 or 1; and A.sup.1 and A.sup.2 each independently represent trans-1,4-cyclohexylene or 1,4-phenylene, PA1 as a second component, containing at least one compound expressed by the formula (2): ##STR3## wherein R.sup.2 represents an alkyl group of 1 to 10C; n represents 0 or 1; Z.sup.1 represents --C.sub.2 H.sub.4 -- or a single bond; A.sup.3 represents trans-1,4-cyclohexylene or 1,4-phenylene; and X.sup.1 represents H, F or Cl, PA1 as a third component, containing at least one compound chosen from among a group of compounds expressed by the formula (3): ##STR4## wherein R.sup.3 represents an alkyl group of 1 to 10C; Z.sup.2 represents --C.sub.2 H.sub.4 -- or a single bond; and X.sup.2 and X.sup.3 each independently represent H or F; PA1 the formula (4): ##STR5## wherein R.sup.4 represents an alkyl group of 1 to 10C; Z.sup.3 and Z.sup.4 each independently represent --C.sub.2 H.sub.4 -- or a single bond; A.sup.4 represents trans-1,4-cyclohexylene or 1,4-phenyl-ene; and X.sup.4 and X.sup.5 each independently represent H or F or PA1 the formula (5): ##STR6## wherein R.sup.5 represents an alkyl group of 1 to 10C; Z.sup.5 represents --C.sub.2 H.sub.4 -- or a single bond; and X.sup.6 and X.sup.7 each independently represent H or F. PA1 3 to 40% by weight of a first component containing at least one compound chosen from among a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compounds expressed by the above formula (2a); and PA1 3 to 70% by weight of a third component containing at least one compound chosen from among a group of compounds expressed by the above formula (4b) and at least one compound chosen from among a group of compounds expressed by the above formula (4c). PA1 3 to 40% by weight of a first component containing at least one compound chosen from among a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compounds expressed by the above formula (2a); and PA1 3 to 70% by weight of a third component containing at least one compound chosen from among a group of compounds expressed by the above formula (3a), and at least one compound chosen from among a group of compounds expressed by the above formula (4c). PA1 3 to 40% by weight of a first component containing at least one compound chosen from among a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compounds expressed by the above formula (2a); and PA1 3 to 70% by weight of a third component containing at least one compound chosen from a group of compounds expressed by the above formula (4a), at least one compound chosen from among a group of compounds expressed by the above formula (4b), and at least one compound chosen from among a group of compounds expressed by the above formula (4c). PA1 3 to 40% by weight of a first component containing at least one compound chosen from among a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compounds expressed by the above formula (2a), at least one compound chosen from among a group of compounds expressed by the above formula (2b), and at least one compound chosen from a group of compounds expressed by the above formula (2c); and PA1 3 to 70% by weight of a third component containing at least one compound chosen from among a group of compounds expressed by the above formula (3a) and at least one compound chosen from a group of compounds expressed by the above formula (4c). PA1 3 to 40% by weight of a first component containing at least one compound chosen from a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compounds expressed by the above formula (2a); and PA1 3 to 70% by weight of a third component containing at least one compound chosen from among a group of compounds expressed by the above formula (3a), at least one compound chosen from a group of compounds expressed by the above formula (4b), and at least one compound chosen from a group of compounds expressed by the formula (4c). PA1 3 to 40% by weight of a first component containing at least one compound chosen from among a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compounds expressed by the above formula (2a); and PA1 3 to 70% by weight of a third component containing at least one compound chosen from among a group of compounds expressed by the above formula (4c), and at least one compound chosen from among a group of compounds expressed by the above formula (5a). PA1 3 to 40% by weight of a first component containing at least one compound chosen from among a group of compounds expressed by the above formula (1a); PA1 3 to 30% by weight of a second component containing at least one compound chosen from among a group of compound expressed by the above formula (2a); PA1 3 to 70% by weight of a third component containing at least one compound chosen from among a group of compounds expressed by the above formula (4c); and PA1 3 to 40% by weight of a fourth component containing at least one compound chosen from among a group of compounds expressed by the above formula (6a), at least one compound chosen from among a group of compounds expressed by the above formula (7a), and at least one compound chosen from among a group of compounds expressed by the above formula (7b).
Among these characteristics, the threshold voltage of 1) is a characteristic required for realizing reduction of the driving voltage of AM-LCD. In recent years, LCD has also been used for note type, personal computer, etc. characterized by being portable in a small type and light weight. These equipments aiming at portable use are restricted by driving electric source; hence it is necessary for further light-weight to make the device for electric source smaller. Thus, LCD, too, should be driven at a lower voltage. Accordingly, as the liquid crystal material, it has been necessary to lower the threshold voltage among electrooptical characteristics.
The specific resistance, i.e. the voltage retention of the above 2) is a characteristic required for realizing an AM-LCD having a high contrast and a high reliability. When the specific resistance, i.e. the voltage retention is low, a display unevenness and contrast reduction occur; hence this has a bad effect upon the reliability of the display element. Heretofore, compounds having a cyano group at its terminal have been used as a component of the liquid crystal composition, because the polarity of the cyano group induces a high dielectric anisotropy in the molecule and this has been regarded as effective for lowering the threshold voltage of the liquid crystal composition. However, as described in Comparative example of Japanese patent application laid-open No. Hei 2-289682, since these compounds lower the specific resistance of liquid crystal composition, they cannot be used for AM-LCD requiring a reliability higher than that of conventional ones. At present, a liquid crystal composition containing no compound having cyano group and exhibiting a higher specific resistance, i.e. a higher voltage-retention at service temperature has been sought.
The optical anisotropy of the above 3) is a characteristic related to the contrast of AM-LCD.
In the case of TN display mode, as reported by G. Bauer in Liq. Cryst, 63, 45 (1981), it is necessary for preventing occurrence of interference fringes on the cell surface, to establish the product of the optical anisotropy (.DELTA.n) of a liquid crystal material filled in a cell by the cell thickness (d .mu.m), .DELTA.n.multidot.d of LDC, to a specified value (for example, .DELTA.n.multidot.d=0.5 .mu.m or .DELTA.n.multidot.d=1.0 .mu.m, etc.).
In short, if .DELTA.n is not an adequate value, there occurs a problem of inferior contrast.
Recently, the liquid crystal composition used for AM-LCD is intended to increase .DELTA.n, in addition to so far sought characteristics, because, this leads to increase in the response speed of AM-LCD. LCD should be capable of being driven at a response speed as fast as possible, and it is well known that the response time of LCD is proportional to the viscosity of liquid crystal and the square of the cell thickness of LCD. Thus, in order to increase the response speed of LCD, it is necessary to decrease the viscosity of the liquid crystal or to shorten the cell thickness of LCD. When the cell thickness is reduced, a liquid crystal composition having a large .DELTA.n is required. When such a liquid crystal composition is used, it is possible to reduce the cell thickness of LCD, and shorten the response time.
Japanese patent application laid-open No. Hei 2-289682 discloses liquid crystal compositions having the object of, in addition to the characteristics of low current consumption and high specific resistance, characteristics of low viscosity and suitable, positive dielectric anisotropy, and further, to afford a liquid crystal display element having a high reliability and a high display contrast. These liquid crystal compositions have a high specific resistance i.e. a high voltage retention at service temperature and hence can be used for AM-LCD, but have a drawback that the threshold voltage is high, so that they are insufficient to be used for portable AM-LCD.
As a liquid crystal composition having overcome the drawback of the liquid crystal compositions disclosed in the above Japanese patent application laid-open No. Hei 2-289682, a liquid crystal composition having a relatively low viscosity and a low threshold voltage, while retaining a high specific resistance and a low current consumption, and a liquid crystal display device using the above liquid crystal composition and having a high contrast, a high reliability, a relatively short response time and effecting a low voltage drive, is disclosed in WO94/03558. This liquid crystal composition has characteristics of a high voltage retention at service temperature and a low threshold voltage, but it has a drawback that since the optical anisotropy is small, it is impossible to reduce the cell thickness of LCD and hence it cannot satisfy to the requirement of further raising the response rate.
As described above, it is the present status that as a liquid crystal required for the present AM-LCD, one which is driven at a low voltage, while retaining a high voltage retention, and has a suitably large optical anisotropy has been earnestly desired.
The object of the present invention is to provide a liquid crystal composition which is reduced in the threshold voltage, while retaining a high voltage retention at service temperature, and has a suitably large optical anisotropy.